1. Field of the Invention
The present invention relates generally to anesthesia machines and, more particularly, to an anesthesia machine having a pressure relief valve with vent override capabilities. The present invention also relates to an anesthesia machine having a manifold with modular components, including a carbon dioxide absorber that is easy to clean and reseal.
2. Discussion of the Related Art
Anesthesia machines typically include a substantially closed breathing system that provides an oxygen/anesthesia gas mixture to the patient for inhalation and receives and processes exhaled gas from the patient for recirculation. The breathing system processes the exhaled gas by removing carbon dioxide from the gas and merges this processed exhaled gas with the flow of the oxygen/anesthesia gas mixture. The breathing system interfaces with the patient via inhalation and exhalation tubes connected to a breathing mask or other patient interface.
The breathing system is not completely closed since fresh anesthesia gas and oxygen are continually supplied to the breathing system from an external source. Therefore, a pressure relief or pop-off valve communicates with the breathing system and evacuates excess gas from the breathing system to compensate for the fresh gas entering the system when pressure in the system exceeds a predetermined level. The vented gas is preferably collected by a closed vacuum system rather than released to the atmosphere to avoid exposure of the gas to operating room personnel.
Conventional pressure relief valves typically include a valve closure element that is spring biased against a valve seat. When a predetermined pressure is applied beneath the valve closure, it is lifted from the seat and gas is vented from the breathing system through an outlet port. The pressure threshold at which the valve opens is adjustable by varying the biasing force on the valve, which is usually accomplished by rotating an adjustment knob threadably secured to a valve housing to selectively compress the bias spring against the valve closure element.
The breathing system also typically includes an expandable member, such as a breathing bag, for receiving gas exhaled by the patient. The bag inflates during exhalation and deflates during inhalation. If the patient stops breathing, the bag may be squeezed to force gas into the patient's lungs in order to stimulate breathing. The pressure relief valve must be fully closed during this procedure, referred to as "bagging," so that gas flows into the patient's lungs rather than escaping through the pressure relief valve.
Since "bagging" is usually performed in emergency situations, reaction time is critical; therefore the pressure relief valve must be quickly closed. As a result, the adjustment knob is coarsely threaded and usually requires only three rotations to move between maximum and minimum bias positions.
The pressure relief valve must also be able to precisely set the pressure threshold for the breathing system. Coarsely threaded adjustment knobs, however, sacrifice precision and it can be quite troublesome to precisely achieve the desired pressure threshold. In fact, users commonly forego setting the pressure threshold with the adjustment knob, and instead set the valve to the minimum bias position (i.e., minimal pressure required to unseat the valve member) and increase the oxygen flow as much as three times the normal requirement. While this practice may be sufficient, it greatly increases consumption costs since tripling the oxygen flow rate also triples the anesthesia consumption rate. Considering that an anesthetic such as isofluorine costs about $65 per 3 oz., such a practice is quite costly.
Finely threaded adjustment knobs provide more precision since a slight turn of the adjustment knob will only vary the pressure setting by a small increment. However, they require five or six turns to adjust the spring to maximum bias, thus wasting valuable time when "bagging" is necessary.
In addition to the above drawbacks, the knob, regardless of whether it is finely-threaded or coarsely-threaded, must be rotated from the maximum bias position back to the previous bias setting after "bagging" to reset the pressure threshold for the system. As a result, additional time and effort is required.
In light of the above drawbacks, there is a significant need for a pressure relief valve that can be precisely adjusted to a desired pressure threshold setting and can be quickly positioned to prevent venting and then quickly returned to the desired pressure threshold setting.
Conventional anesthesia machines also suffer from disadvantages in that the components of the breathing system, e.g., check valves, a manometer, flowmeters, a carbon dioxide absorber, and a pressure relief valve, are typically interconnected with several segments of flexible tubing or rigid conduits having bends or other configurations that are difficult to clean. As a result, substantial time and effort is required to thoroughly clean the breathing system. Otherwise, contaminants may remain in the system, thus posing risk to the patient.
In addition to cleaning the flow paths of the breathing system, individual components may require disassembly for cleaning. For example, the carbon dioxide absorber, which generally includes a cannister and lid, must be cleaned by removing the carbon dioxide absorbing material (e.g., calcium hydroxide) from the cannister and replacing it with a fresh supply. Since a tight seal between the lid and cannister must be maintained during use, it is imperative that the seal between the lid and cannister be effectively reestablished. Since conventional designs render it difficult to correctly and easily align the lid with the cannister, the integrity of the seal and the breathing system is jeopardized.